[test] small string compatibility for 32-bit targets

Author: glessard

test/stdlib/Span/SmallStringCompatibility.swift

@@ -0,0 +1,67 @@
+//===--- SmallStringCompatibility.swift -----------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+// RUN: %target-run-stdlib-swift
+
+// REQUIRES: executable_test
+// REQUIRES: CPU=wasm32 || CPU=arm64_32
+
+import StdlibUnittest
+
+var suite = TestSuite("SmallStringCompatibility")
+defer { runAllTests() }
+
+var strings = [
+  ("Small", true),
+  ("Less small", true),
+  ("Positively large.", true),
+]
+
+#if os(watchOS)
+strings[1].1 = false
+#endif
+
+strings.forEach { (string, contiguous) in
+  suite.test("Contiguous: \(string)")
+  .require(.stdlib_6_2).code {
+
+    expectEqual(string.isContiguousUTF8, contiguous)
+  }
+}
+
+strings.forEach { (string, contiguous) in
+  suite.test("Contiguous Substring: \(string)")
+  .require(.stdlib_6_2).code {
+    let substring = string[...]
+    expectEqual(substring.isContiguousUTF8, contiguous)
+  }
+}
+
+strings.forEach { (string, contiguous) in
+  suite.test("String.makeContiguousUTF8: \(string)")
+  .require(.stdlib_6_2).code {
+    var s = string
+    s.makeContiguousUTF8()
+    expectTrue(s.isContiguousUTF8)
+    expectEqual(s, string)
+  }
+}
+
+strings.forEach { (string, contiguous) in
+  suite.test("Substring.makeContiguousUTF8: \(string)")
+  .require(.stdlib_6_2).code {
+    var s: Substring = string.dropFirst().dropLast()
+    s.makeContiguousUTF8()
+    expectTrue(s.isContiguousUTF8)
+    expectEqual(s, string.dropFirst().dropLast())
+  }
+}